The preparation of cyclic esters of alpha-hydroxycarboxylic acids is an old and much studied process. Heretofore, the preparation has been conducted in two distinct batch steps involving first preparing an oligomer of the hydroxycarboxylic acid, i.e., a relatively short-chain condensation polymer thereof, then heating the polymer under reduced pressure to generate the desired cyclic ester. Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,162 (1954); Bhatia, U.S. Pat. No. 4,835,293 (1989); Bellis U.S. Pat. No. 4,727,163 (1988); Muller, Ger. Pat. Applications 3632103 and 3708915 (1988). Such processes spanning over 70 years of technology suffer in that they require hours of reaction time at high temperatures for the preparation of the polymeric intermediate and its thermolysis to the cyclic ester. Further, the rather long residence times at the high temperatures employed often results in side reactions, leading, for example, to unwanted isomers, charring of the polymer and consequently difficult to handle reactor heels.
It is an object of this invention to provide a novel essentially single-step process for the rapid conversion of an alpha-hydroxycarboxylic acid, such as lactic acid, to a cyclic ester, such as lactide.
Another object is to provide such a process that enables continuous production of a cyclic ester such as lactide directly and rapidly from the corresponding alpha-hydroxycarboxylic acid in a single reaction zone without need to separately prepare an oligomer of the acid as intermediate to the cyclic ester.